Cosmetic fixes to clobber_generated_headers.py:...

Cosmetic fixes to clobber_generated_headers.py:
Only try to remove a generated header file if it exists.
Only report "Clobbered" if we actually removed something.
Print the actual system error if the removal fails.
Print the error message on stderr, not stdout.
Normalize the header file path name.
BUG=none
TEST=none

[sgk, 2009-08-07 19:58:42]

Suggested changes to the new clobber_generated_headers.py hook's reporting: 
basically, quieter about normal operation.

[Paweł Hajdan Jr., 2009-08-07 20:01:51]

On 2009/08/07 19:58:42, sgk wrote:
> Suggested changes to the new clobber_generated_headers.py hook's reporting: 
> basically, quieter about normal operation.

Please delay committing this change until I get the headers paths right. I'm
working with maruel on that. The verbose output helps with making sure the
script tries to remove the right file.

Feel free to check-in the path normalization code.

[sgk, 2009-08-07 20:07:13]

If you're working on more changes, feel free to fold any of this in as it suits
you.  None of it has to come from this CL, or is even specifically urgent, but
this is how (IMHO) it should eventually behave once you're finished debugging
and putting it in final form.  I'll leave this review open for reference until
you're there.

Thanks,

[Paweł Hajdan Jr., 2009-08-19 18:14:04]

LGTM

People are so annoyed about these warnings, and the script seems to work fine.
Please commit, and thanks for the change.